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record4s — Extensible Records for Scala 3,
and Domain Modeling with Structural Types
Lintaro INA
id:tarao @tarao @oarat
2024-06-08
ScalaMatsuri 2024
record4s — Scala 3 ͷͨΊͷ֦ுՄೳϨίʔυɺ
ͦͯ͠ߏతܕʹΑΔυϝΠϯϞσϦϯά
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Are you tired of
defining similar models?
Example: a few difference
case class BlogPost(
id: PostId,
title: String,
body: String,
url: URL,
publishedAt: DateTime,
)
case class BlogPostWithPaywall(
id: PostId,
title: String,
body: String,
paidPartOfBody: Option[String],
url: URL,
publishedAt: DateTime,
)
ྫɿ ϒϩάهࣄΛද͢Ϟσϧʢࠨʣͱɺهࣄͷ༗ྉ෦࣋ͭϞσϧʢӈʣ
paidPartOfBody ͷ༗ແ͚ͩҟͳΔ
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Are you tired of
defining similar models?
Example
blogPostRepository.find: PostId => Option[ BlogPost ]
paywallRepository.findPaidBody: PostId => Option[String]
def blogPostWithPaywall(postId: PostId): Option[ BlogPostWithPaywall ] =
blogPostRepository.find(postId).map: post => // get a blog post
BlogPostWithPaywall(
id = post.id,
title = post.title,
body = post.body,
paidPartOfBody = paywallRepository.findPaidBody(post.id), // add paid part of body
url = post.url,
publishedAt = post.publishedAt,
)
ྫɿ ϒϩάهࣄΛऔಘͨ͠ޙɺ༗ྉ෦औಘ͢Δ
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Are you tired of
defining similar models?
Example: getting a BlogPost
blogPostRepository.find: PostId => Option[ BlogPost ]
paywallRepository.findPaidBody: PostId => Option[String]
def blogPostWithPaywall(postId: PostId): Option[ BlogPostWithPaywall ] =
blogPostRepository.find(postId).map: post => // get a blog post
BlogPostWithPaywall(
id = post.id,
title = post.title,
body = post.body,
paidPartOfBody = paywallRepository.findPaidBody(post.id), // add paid part of body
url = post.url,
publishedAt = post.publishedAt,
)
ྫɿ ϒϩάهࣄΛऔಘͨ͠ޙɺ༗ྉ෦औಘ͢Δ
·ͣ BlogPost ΛಘΔ
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Are you tired of
defining similar models?
Example: adding a field
blogPostRepository.find: PostId => Option[ BlogPost ]
paywallRepository.findPaidBody: PostId => Option[String]
def blogPostWithPaywall(postId: PostId): Option[ BlogPostWithPaywall ] =
blogPostRepository.find(postId).map: post => // get a blog post
BlogPostWithPaywall(
id = post.id,
title = post.title,
body = post.body,
paidPartOfBody = paywallRepository.findPaidBody(post.id), // add paid part of body
url = post.url,
publishedAt = post.publishedAt,
)
ྫɿ ϒϩάهࣄΛऔಘͨ͠ޙɺ༗ྉ෦औಘ͢Δ
paidPartOfBody Λͯ͠ BlogPostWithPaywall ʹ͢Δ
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Are you tired of
defining similar models?
Example: boring passing around
blogPostRepository.find: PostId => Option[ BlogPost ]
paywallRepository.findPaidBody: PostId => Option[String]
def blogPostWithPaywall(postId: PostId): Option[ BlogPostWithPaywall ] =
blogPostRepository.find(postId).map: post => // get a blog post
BlogPostWithPaywall(
id = post.id,
title = post.title,
body = post.body,
paidPartOfBody = paywallRepository.findPaidBody(post.id), // add paid part of body
url = post.url,
publishedAt = post.publishedAt,
)
ྫɿ ϒϩάهࣄΛऔಘͨ͠ޙɺ༗ྉ෦औಘ͢Δ
BlogPost ͔Β BlogPostWithPaywall ͷϑΟʔϧυͷ٧Ίସ͕͑໘
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Are you tired of
defining similar models?
Example
product owner: “Hey, we need draft posts, with paywall of course!”
case class DraftPost(
id: PostId,
title: String,
body: String,
previewUrl: URL,
)
case class DraftPostWithPaywall(
id: PostId,
title: String,
body: String,
paidPartOfBody: Option[String],
previewUrl: URL,
)
ྫɿ ϓϩμΫτΦʔφʔʮԼॻ͖ػೳ͕΄͍͠Μ͚ͩͲɻ༗ྉ෦ѻ͑Δͭʂʯ
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Are you tired of
defining similar models?
Example: writing the same thing
draftPostRepository.find: PostId => Option[DraftPost]
paywallRepository.findPaidBody: PostId => Option[String]
def draftPostWithPaywall(postId: PostId): Option[DraftPostWithPaywall] =
draftPostRepository.find(postId).map: post =>
DraftPostWithPaywall(
id = post.id,
title = post.title,
body = post.body,
paidPartOfBody = paywallRepository.findPaidBody(post.id),
previewUrl = post.previewUrl,
)
ྫɿ ಉ͡Α͏ͳ͜ͱΛॻ͘Ӌʹ
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Are you tired of
defining similar models?
Example
product owner: “Hey, we need modification date, for both
published and draft posts!”
ྫɿ ϓϩμΫτΦʔφʔʮߋ৽͕࣌΄͍͠Μ͚ͩͲɻެ։ࡁΈهࣄͱԼॻ͖྆ํʂʯ
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Are you tired of
defining similar models?
Example
product owner: “Hey, we need modification date, for both
published and draft posts!”
ྫɿ ϓϩμΫτΦʔφʔʮߋ৽͕࣌΄͍͠Μ͚ͩͲɻެ։ࡁΈهࣄͱԼॻ͖྆ํʂʯ
ͥΜͿॻ͖͑Δͷ໘ʜ
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Wait, we have structural types in Scala 3, right?
ͬͯɺ Scala 3 ʹߏతܕ͕͋Δɺͦ͏Ͱ͠ΐʁ
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Modeling with structural types
Typing in Scala 3
type Model = ... // just some type
type AbstractBlogPost = Model {
val id: PostId
val title: String
val body: String
}
type BlogPost = AbstractBlogPost & Model {
val url: URL
val publishedAt: DateTime
}
type Paid = Model { val paidPartOfBody: Option[String] }
type BlogPostWithPaywall = BlogPost & Paid
type DraftPost =
AbstractBlogPost & Model { val previewUrl: URL }
type DraftPostWithPaywall = DraftPost & Paid
▶ Listing field types by val
▶ Merging field sets by &
▶ No repeat
val ͱ &ͰܕΛఆٛ͢Δ / ಉ͜͡ͱΛ܁Γฦ͠ॻ͔ͳ͍ͰࡁΉ
͜Ε Scala 3 ͷܕγεςϜͷػೳ
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Modeling with structural types
A way described in Scala 3 Book
class Model(elems: (String, Any)*) extends Selectable:
val fields = elems.toMap
def selectDynamic(name: String): Any = fields(name)
val post = Model(
"id" -> postId,
"title" -> title,
"body" -> body,
"url" -> url,
"publishedAt" -> publishedAt,
). asInstanceOf[BlogPost]
ߏతܕͷΦϒδΣΫτͷఆٛͷ͔ͨ͠ެࣜ Scala 3 Book ʹઆ໌͕͋Δ
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Modeling with structural types
A way described in Scala 3 Book
class Model(elems: (String, Any)*) extends Selectable:
val fields = elems.toMap
def selectDynamic(name: String): Any = fields(name)
val post = Model(
"id" -> postId,
"title" -> title,
"body" -> body,
"url" -> url,
"publishedAt" -> publishedAt,
). asInstanceOf[BlogPost]
▶ Unsafe asInstanceOf
ߏతܕͷΦϒδΣΫτͷఆٛͷ͔ͨ͠ެࣜ Scala 3 Book ʹઆ໌͕͋Δ
ͨͩ͠ɺ asInstanceOf ͕ඞཁͰΠέͯͳ͍
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Modeling with structural types
A way described in Scala 3 Book
class Model(elems: (String, Any)*) extends Selectable:
val fields = elems.toMap
def selectDynamic(name: String): Any = fields(name)
val post = Model(
"id" -> postId,
"title" -> title,
"body" -> body,
"url" -> url,
"publishedAt" -> publishedAt,
). asInstanceOf[BlogPost]
val fields =
post.fields + ("paidPartOfBody" -> paidPartOfBody)
val postWithPaywall =
Model(fields.toSeq*)
.asInstanceOf[BlogPostWithPaywall]
▶ Unsafe asInstanceOf ▶ Adding a field by hand
ߏతܕͷΦϒδΣΫτͷఆٛͷ͔ͨ͠ެࣜ Scala 3 Book ʹઆ໌͕͋Δ
ͨͩ͠ɺ asInstanceOf ͕ඞཁͰΠέͯͳ͍ / ϑΟʔϧυͷՃ͕ແཧΓ
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Modeling with structural types
A way described in Scala 3 Book
class Model(elems: (String, Any)*) extends Selectable:
val fields = elems.toMap
def selectDynamic(name: String): Any = fields(name)
val post = Model(
"id" -> postId,
"title" -> title,
"body" -> body,
"url" -> url,
"publishedAt" -> publishedAt,
). asInstanceOf[BlogPost]
val fields =
post.fields + ("paidPartOfBody" -> paidPartOfBody)
val postWithPaywall =
Model(fields.toSeq*)
.asInstanceOf[BlogPostWithPaywall]
▶ Unsafe asInstanceOf ▶ Adding a field by hand
ߏతܕͷΦϒδΣΫτͷఆٛͷ͔ͨ͠ެࣜ Scala 3 Book ʹઆ໌͕͋Δ
ͨͩ͠ɺ asInstanceOf ͕ඞཁͰΠέͯͳ͍ / ϑΟʔϧυͷՃ͕ແཧΓ
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Modeling with structural types
Imagine that instantiation goes straightforward
An ideal way of instantiation
val post: BlogPost = Model(
id = postId,
title = title,
body = body,
url = url,
publishedAt = publishedAt
)
ૉʹΠϯελϯεԽ͍ͨ͠
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Modeling with structural types
Imagine that objects can be extended
An ideal way of adding a field
val postWithPaywall: BlogPostWithPaywall =
post + (paidPartOfBody = paidPartOfBody)
ΦϒδΣΫτʹϑΟʔϧυΛ֦ͯ͠ு͍ͨ͠
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Are these possible?
ͦΜͳ͜ͱ͕Ͱ͖ͪΌ͏
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Are these possible?
With record4s, Yes!
ͦΜͳ͜ͱ͕Ͱ͖ͪΌ͏
ͦ͏ɺ record4s ͳΒͶʂ
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tarao/record4s
Extensible records for Scala
github.com
Try now with Scala CLI:
//> using dep "com.github.tarao::record4s:0.12.0"
import com.github.tarao.record4s.%
The previous examples work with:
import com.github.tarao.record4s.%
type Model = %
val Model = %
GitHub Ͱެ։ࡁΈ / Scala CLI Ͱࠓ͙͢ࢼͦ͏ʂ
લͷྫ Model ΛΤΠϦΞεʹ͢Δͱͦͷ··ಈ͘
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What record4s can do
Record construction val person = %(
name = "tarao",
age = 3,
)
// person: % {
// val name: String
// val age: Int
// } = %(name = tarao, age = 3)
record4s ͰͰ͖Δ͜ͱ
Ϩίʔυͷ࡞ɿ ߏతܕͷΦϒδΣΫτ͕࡞ΒΕΔ
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What record4s can do
Record construction
Type-safe field access
person.name
// res0: String = "tarao"
person.age
// res1: Int = 3
person.email
// error:
// value email is not a member of
// %{val name: String; val age: Int}
record4s ͰͰ͖Δ͜ͱ
ܕ҆શͳϑΟʔϧυΞΫηεɿ ଘࡏ͠ͳ͍ϑΟʔϧυίϯύΠϧΤϥʔʹ
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What record4s can do
Record construction
Type-safe field access
Extension
person + (
email = "[email protected]",
occupation = "engineer",
)
// res2: % {
// val name: String
// val age: Int
// val email: String
// val occupation: String
// } = %(name = tarao, age = 3,
// email = [email protected],
// occupation = engineer)
record4s ͰͰ͖Δ͜ͱ
Ϩίʔυͷ֦ுʢෳϑΟʔϧυಉ࣌ʹՃՄೳʣ
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What record4s can do
Record construction
Type-safe field access
Extension
Concatenation
val email = %(email = "[email protected]")
// email: % {
// val email: String
// } = %(email = [email protected])
person ++ email
// res3: % {
// val name: String
// val age: Int
// val email: String
// } = %(name = tarao, age = 3,
// email = [email protected])
record4s ͰͰ͖Δ͜ͱ
Ϩίʔυͷ݁߹
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What record4s can do
Record construction
Type-safe field access
Extension
Concatenation
Field update
person + (age = person.age + 1)
// res4: % {
// val name: String
// val age: Int
// } = %(name = tarao, age = 4)
person + (name = %(first = "tarao", last = "fuguta"))
// res5: % {
// val name: % {
// val first: String
// val last: String
// }
// val age: Int
// } = %(name = %(first = tarao, last = fuguta), age = 3)
record4s ͰͰ͖Δ͜ͱ
ϑΟʔϧυͷߋ৽ɿ ผͷܕͰ্ॻ͖ͯ͠Α͍
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Other features
Type tagging
Selecting/unselecting/reordering/renaming fields
Pattern matching
Conversion from/to case classes
JSON conversion (using circe or uPickle)
Scala.js support
Scala Native support
ଞʹ͍Ζ͍Ζػೳ͕๛
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Tips — Methods on records
Methods on records can be defined as extension methods
extension (post: BlogPost)
def summary(length: Int = 200): String = post.body.take(length)
val post: BlogPost = ...
val postWithPaywall: BlogPostWithPaywall = ...
post.summary() // works
postWithPaywall.summary() // also works
// Note: we have BlogPostWithPaywall <: BlogPost
// since BlogPostWithPaywall has more fields
// (That is structural subtyping)
Ϩίʔυͷϝιου֦ுϝιουͱͯ͠ఆٛՄೳ
ߏత෦ܕʹͳ͍ͬͯΔΦϒδΣΫτʹରͯ͠ಉ͡ϝιουΛݺΔ
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Tips — Decoding JSON by example
Normally, you have to specify record type to decode from JSON
import com.github.tarao.record4s.upickle.Record.readWriter
import upickle.default.read
val json = """{"name":"tarao","age":3}"""
type Person = % { val name: String; val age: Int } // you have to write this!
val r = read[Person](json)
௨ৗɺ JSON ͔Βσίʔυ͢ΔʹσίʔυઌͷܕΛॻ͘ඞཁ͕͋Δ
ʢͦͯͦ͠Ε͠͠໘ʣ
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Tips — Decoding JSON by example
Giving an example let you omit the type definition
import com.github.tarao.record4s.upickle.Record.readWriter
import upickle.default.{Reader, read}
def decodeByExample[R <: `%`: Reader](json: String, example: R): R =
read[R](json)
val json = """{"name":"tarao","age":3}"""
val example = %(name = "ikura", age = 1) // no type definition
val r = decodeByExample(json, example)
۩ମྫΛ༩͑ΕܕఆٛলུՄೳ
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Internals
෦࣮
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Field access
1. Use Selectable from the standard library
abstract class % extends Selectable:
def __data: Map[String, Any]
def selectDynamic(name: String): Any = __data(name)
final class MapRecord(val __data: Map[String, Any]) extends %
2. Field access compiles to selectDynamic call
val r = %(name = "tarao", age = 3) // returns a MapRecord
r.age // ⇝ r.selectDynamic("age").asInstanceOf[Int]
1. ඪ४ϥΠϒϥϦͷ Selectable Λ͏
2. ϑΟʔϧυΞΫηείϯύΠϧ࣌ʹselectDynamic ͷݺͼग़͠ʹஔ͞ΕΔ
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Concatenation
val r1 = %(...)
val r2 = %(...)
r1 ++ r2
inlining
⇝
val r1: R1 = %(...)
val r2: R2 = %(...)
val c = summon[Concat[R1, R2]]
new MapRecord(r1.__data.concat(r2.__data))
.asInstanceOf[c.Out]
Ϩίʔυͷ݁߹ Map ͷ concat ʹ͢Δ
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Concatenation
val r1 = %(...)
val r2 = %(...)
r1 ++ r2
inlining
⇝
val r1: R1 = %(...)
val r2: R2 = %(...)
val c = summon[Concat[R1, R2]]
new MapRecord(r1.__data.concat(r2.__data))
.asInstanceOf[c.Out]
// transparent macro to calculate Out from R1 and R2
transparent inline given [R1, R2]: Concat[R1, R2] =
${ Macros.derivedTypingConcatImpl }
Ϩίʔυͷ݁߹ Map ͷ concat ʹ͢Δ
Concat given ͕ϚΫϩͰ࣮͞Ε͍ͯͯɺ݁߹݁ՌͷܕΛܭࢉ͢Δ
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Concatenation
val r1 = %(...)
val r2 = %(...)
r1 ++ r2
inlining
⇝
val r1: R1 = %(...)
val r2: R2 = %(...)
val c = summon[Concat[R1, R2]]
new MapRecord(r1.__data.concat(r2.__data))
.asInstanceOf[c.Out]
Wait, is this really safe?
Ϩίʔυͷ݁߹ Map ͷ concat ʹ͢Δ
ͱ͜ΖͰ͜Εຊʹ҆શʁ
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Key duplication problem
val r1 = %(age = 3)
val r2: % { val name: String } = %(name = "tarao", age = "unknown")
// legal since % { val name: String; val age: String } <: % { val name: String }
val r = r1 ++ r2
// r: % {
// val age: Int
// val name: String
// } = %(age = "unknwon", name = "tarao")
r.age // java.lang.ClassCastException:
// class java.lang.String cannot be cast to class java.lang.Integer
݁߹ͷӈลͷ੩తܕʹݱΕͳ͍ϑΟʔϧυ͕ඇޓͳܕͰ্ॻ͖Ͱ͖ͯ͠·͏
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Key duplication problem
val r1 = %(age = 3)
val r2: % { val name: String } = %(name = "tarao", age = "unknown")
// legal since % { val name: String; val age: String } <: % { val name: String }
val r = r1 ++ r2
// r: % {
// val age: Int
// val name: String
// } = %(age = "unknwon", name = "tarao")
r.age // java.lang.ClassCastException:
// class java.lang.String cannot be cast to class java.lang.Integer
We need to “tidy up” RHS value by its static type
݁߹ͷӈลͷ੩తܕʹݱΕͳ͍ϑΟʔϧυ͕ඇޓͳܕͰ্ॻ͖Ͱ͖ͯ͠·͏
ӈล੩తܕʹ ἧ͓͑ͯ͘ඞཁ͕͋Δ
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Key duplication problem
Something like this:
val r1 = %(...)
val r2 = %(...)
r1 ++ r2
inlining
⇝
val r1: R1 = %(...)
val r2: R2 = %(...)
val c = summon[Concat[R1, R2]]
new MapRecord(r1.__data.concat(r2.__data))
.asInstanceOf[c.Out]
͜͜Λ
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Key duplication problem
Something like this:
val r1 = %(...)
val r2 = %(...)
r1 ++ r2
inlining
⇝
val r1: R1 = %(...)
val r2: R2 = %(...)
val c = summon[Concat[R1, R2]]
new MapRecord(r1.__data.concat(r2.as[R2].__data))
.asInstanceOf[c.Out]
ͳΜΒ͔͜ͷΑ͏ʹ
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Performance characteristics
ύϑΥʔϚϯεಛੑ
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Runtime performance
Creation time
▶ Grows linearly against record
size
▶ Almost the same as Map
Ϩίʔυͷ࡞࣌ؒαΠζʹରͯ͠ઢܗ
Map ͱಉ
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Runtime performance
Field access time
▶ Effectively constant time
ϑΟʔϧυΞΫηε࣮࣭ఆ࣌ؒ
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Compile-time performance
Compilation time of concatenation
▶ Grows quadratically against
record size
▶ Due to subtyping check
of structural types
▶ But far better than
shapeless Record
Ϩίʔυ݁߹ͷίϯύΠϧ࣌ؒαΠζͷೋʹൺྫʢߏత෦ܕͷݕࠪͷͨΊʣ
ͱ͍͑ shapeless ΑΓͣͬͱ͍͍
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Conclusion
record4s provides extensible records for Scala 3
It is useful for some kind of domain modeling
It is carefully designed using inlining and macros
It has good enough performance characteristics
record4s Scala 3 ͚ͷ֦ுՄೳϨίʔυΛఏڙ͠ɺυϝΠϯϞσϦϯάʹཱͭ
ϚΫϩΛ༻͍ͯҙਂ࣮͘͞Ε͍ͯͯɺेͳύϑΥʔϚϯεಛੑΛඋ͍͑ͯΔ